After (accidentally) entering bootloader mode on my nRF51 DK (PCA10028), the device doesnt show up either in the device manager or using nRFgo Studio (both not normally, as being in the bootloader mode where LD5 is blinking.)

In case I turn the DK on and off normally (without pressing the reset button), the LD5 doesnt blink or turn on otherwise. If I turn the DK on while holding reset, BOOTLOADER shows up as a USB device, and the LD5 blinks.

1 answer

Enter bootloader mode and drag and drop a debugger firmware image onto the drive.
The image can be found at nRF5x-OB-JLink-IF.
After programming is done, the green debug led should start flashing rapidly. After power cycling your board it should be back to normal.

The DKs come preprogrammed with the firmware image. I'm guessing you somehow did something to erase or overwrite this when you entered bootloader mode by accident. This can happen if you tried to program the nrf52 with the BOOTLOADER drive for instance.